Coex

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Note: For the convention and exhibition center in Seoul, Korea, see COEX Convention & Exhibition Center.

The CoEx is the name of a brewer in a espresso machine, coffee machine or vending machine which is able to brew a cup of coffee as well as an espresso. It is a mechanism in which with ground coffee and hot water (on pressure in case of espresso), a cup of coffee or espresso can be made, and from which, after the drink has been prepared the coffee residue is removed.

1 The mechanism
2 Coffee or espresso
3 Capacities
4 External links

[edit] The mechanism

Cutaway diagram of a Coex mechanism

Firstly ground coffee is dosed into the cylinder chamber. Then the cylinder chamber is closed by the upper piston, which moves above the chamber then down to seal it. Hot water is passed through under pressure, coming from the back of the mechanism via the lower piston, through the ground coffee in the cylinder chamber. The brewed coffee (or espresso) leaves the system through a filterscreen in the upper piston, at the front of the system. After brewing the upper piston is pulled out of the cylinder and moved to one side. The lower piston then moves upwards, pushing the coffee residue out of the cylinder chamber. In the last phase the coffee residue is wiped from the brewer toward a waste bucket next to the brewer and the mechanism returns to its start position.

[edit] Coffee or espresso

The mechanism distinguish between the way in which coffee or espresso is being prepared. The coffee, which is supposed to be brewed with coarse grinded, lightly burned coffee beans, is made on a low pressure of 1[bar], whereas espresso, which is supposed to be brewed with a fine grind, strongly burned coffee beans, is made on a high pressure of 9-10[bar]. The difference in pressure is created by a mechanical switch in the upper piston. As long as the pressure remains low, the force of the spring will keep the switch in place and the holes will remain open. If the pressure increases, the switch will move up and closes the big hole. The pressure will then increase quickly to the pressure level the pumps are able to deliver. (Those pump are not part of the brewing system, but delivers the hot water under low or high pressure to the system.)
In the schematic pictures at the right, those differences are shown. In the picture above, the water pressure in the cylinder filled with coffee is low and the spring is able to keep the switch open. The coffee is able to leave the system fluently, trough a filter and through a relatively big hole. In the picture below the water pressure was able to move the switch to the upper position. The big hole is closed and the pressure increases further to the pressure level the pumps are able to deliver. Under pressure the espresso leaves the brewer via the small remaining hole.